Transforming growth factor-β (TGF-β) is a cytokine crucial for regulating diverse aspects of cellular homeostasis. TGF-β signals are transduced into the cell by the Smad transcription factors, which accumulate in the nucleus during signaling and drive the regulation of TGF-β target gene expression. These genes then cooperate to mediate the cell response. The quantitative basis for how TGF-β signals are transduced into Smad kinetics is poorly understood. My thesis is therefore devoted to the quantitative analysis of TGF-β/Smad signaling dynamics. First, I constructed and analyzed a mathematical model of intracellular Smad dynamics. I found that the balance between the Smad phosphorylation rate in the cytoplasm and the Smad dephosphorylation rate in the nucleus determines the degree of Smad nuclear accumulation. Second, I performed a combined experimental and theoretical analysis of how cells read TGF-β concentration. I found that TGF-β dose expressed as TGF-β molecules per cell best predicts phospho-Smad2 levels at both short and long durations. The cellular dependence of TGF-β potency is due largely to the cells continually depleting TGF-β from their culture medium during signaling. Two mechanisms are responsible for TGF-β depletion: (1) a TβRII-dependent process and (2) reversible binding of TGF-β to the cell surface. Continual depletion of TGF-β provides a mechanism for terminating TGF-β signaling, as the duration of Smad phosphorylation correlates with the duration of TGF-β presence in the medium. Neither receptor loss nor regulation of the phospho-Smad2 dephosphorylation rate during signaling account for the reduced phospho-Smad2 levels that accompanies prolonged signaling, thus providing evidence that TGF-β depletion is the causal mechanism for terminating TGF-β signaling. I integrated my data into a new mathematical model of upstream TGF-β signaling, which provides quantitative insight into how TGF-β concentration is transduced into phospho-Smad kinetics.